1. Field of the Invention
The present invention relates to a moisture and dew-detection sensor capable of detecting a change of atmospheric humidity or dew condensation on a surface of a substance.
2. Description of the Prior Art
Dew condensation is a problem in the fields of a precision electrical devices, automobiles, air conditioning systems, warehouses and domestic equipment, etc., and there is a strong demand for a sensor able to accurately detect a high humidity or dew condensation state.
For example, when dew condensation is generated on a cylinder of a rotary head for recording or playing back a magnetic tape in a video tape recorder, a digital audio tape deck or the like, roll-in or a breakage of the magnetic tape, on damage to a device may occur, and therefore various types of dew condensation sensors are arranged in the device.
A dew condensation sensor comprising a moisture sensing resistive substance in which electric conductive powders are dispersed in a moisture absorption high polymer having characteristic of an expansion of a volume thereof by moisture absorption, and a comb-shaped electrode formed on an isolated base plate and covered by the moisture sensing resistive substance in a state of a film having a thickness of from 5 .mu.m to several tens of .mu.m was disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-2110511 or the like, and changes of a resistance value of this dew condensation sensor are large in a high humidity region, to thereby detect a high humidity state and a state in which dew condensation is easily generated, i.e., a state approaching a relative humidity of 100%.
Nevertheless, since the comb-shaped electrode is arranged just below a thin film of the moisture sensing resistive substance, to bring the sensitivity of the sensor upto a practical level, in this type of the dew condensation sensor, when a large quantity of dew is generated by dew condensation, the amount of water which a portion of the film of the moisture sensing resistive substance positioned just below the dew can absorb is immediately exceeded, and excess water not absorbed by the film remains on the surface of the film as water. When the dew condensation is dissolved, the speed of evaporation of moisture in the film is different at a portion of the film covered by the water and than at a portion of the film not covered by the water, so that the moisture in the portion of the film not covered by the water was first evaporated and the value of the resistance of the dew-detection sensor reduced. Therefore this type of the dew-detection sensor caused an error in the detection of humidity when dissolving the dew. Further, when the amount of dew condensation is small, the speed of response of the sensor is slow. To obtain a film having a uniform thickness in this dew-detection sensor, the film must be thick. Water condensed on a surface of the film is absorbed into this film, and thus some time is required to evaporate the water from inside the film, and unavoidably, the response speed when a generation or a dissolving of dew condensation is detected is too slow.
Japanese Unexamined Patent Publication (Kokai) No. 59-43345 disclosed a dew-detection sensor having a detection portion constituted by electric conductive particles provided uniformly on a surface of a moisture absorption fiber. In this dew-detection sensor, the moisture absorption fiber contracts or expands with an absorbent or an adhesive of a moisture adsorption used to make the moisture absorption sensor by change of the humidity. Therefore, when a large amount of water is generated by the dew condensation, the pater is absorbed sequentially into the fiber, and even if a dew condensing state in an atmosphere is eliminated, the dew-detection sensor still maintains the dew condensing state thereof. In particular, when a fiber of a cellulose group is used, since a retention of water molecules caused by a hydrogen bond in the fiber is strong, the fiber is not easily dried, and thus it is impossible to perform an accurate detection of the dew condensation.